1. Field of the Invention
The present invention relates to an optical storage medium, and more particularly to a super-resolution optical medium using an initiation-free layer.
2. Description of the Prior Art
Optical storage media (such as CD, CD-R, CD-RW, and DVD) uses a laser beam irradiating through an objective lens onto a recording layer for optical reading or writing. A recorded mark size is limited by the diffraction limit depending on the wavelength (λ) of the laser beam and the numerical aperture (NA) of the lens, that is, 0.61 λ/NA. Therefore, in order to effectively minimize the mark size to enhance the recording density of the optical storage medium, one can use a light source with a shorter wavelength or increase the numerical aperture of the lens. However, when the optical head develops to DVD-R (digital versatile disc recordable) medium (λ=405 nm, NA=0.85), the wavelength of the laser beam cannot be further shortened, and the NA of the lens cannot be further increased.
In order to further enhance the recording density of the optical storage medium, the near-field optical recording technique has been proposed, for example, the near-field solid immersion lens (SIL) method and the scanning near-field optical microscope (SNOM) method. However, the above-mentioned near-field optical player suffers from difficult fabrication, slow data reading speed, and lack of player portability.
In view of the above drawbacks, Junji Tominaga in 1998 published “Super-resolution Structure for Optical Data Storage by Near-Field Optics”, J. Tominaga, T. Nakano, and N. Atoda, Proc. SPIE 3467, 282 (1998) and then “The Characteristics and the Potential of Super Resolution Near-Field Structure”, J. Tominaga, H. Fuji, A. Sato, T. Takanno, T. Fukaya, N. Atoda, Jpn. J. Appl. Phys. 39, 957 (2000). The research provided a breakthrough in the near-field optical recording technique, described below.
FIG. 1 shows a cross-section of a conventional super-resolution optical near-field structure (“super-RENS”) using silver oxide (AgO). The super-resolution optical medium includes, in sequence, a pre-grooved polycarbonate substrate 10, a ZnS—SiO2 dielectric layer 12 with a thickness of about 130 nm, a AgO active layer 14 with a thickness of about 15 nm, a ZnS—SiO2 dielectric layer 16 with a thickness of about 40 nm, an amorphous Ge2Sb2Te5 alloy recording layer 18 with a thickness of about 20 nm, and a ZnS—SiO2 dielectric layer 20 with a thickness of about 20 nm. When reading, the active layer 14 can absorb the laser beam to form near-field optical effect and generate surface plasma, that is, form a super-resolution optical structure.
The super-resolution principle uses the active layer to absorb the laser beam. When the laser beam passes through the active layer, the optical near-field strength is enhanced by the surface plasma of the active layer. Thus, a very small recorded mark size, even less than the optical diffraction limit, is obtained. In addition, non-linear change on optical properties can also be used. When the laser beam is focused on the active layer, the energy distribution is Gauss distribution, and the rotational substrate causes uneven temperature distribution on the active layer. Also, since the transmittal of the active layer is dependent on the temperature in non-linear relationship, different transmittances for the incident laser beam result at various temperatures. Therefore, the light intensity distribution on the active layer and the light intensity distribution on the recording layer are different. Transmittance change with different light strength distributions can decrease the exposure area of the recording layer, thus achieving super resolution.
In order to improve the carrier to noise ratio (CNR), the as-produced medium must be subjected to a so-called initiation process, in which the recording layer 18 is converted from amorphous to crystalline state by thermal energy. However, the conventional super-resolution optical medium has problems in the initiation process. For example, temperatures as high as 200° C. will damage the active layer 14 of the super-resolution near-field optical medium. Moreover, the initiation facility is expensive and the initiation process is very time-consuming.